1. Field of the Invention
The present invention relates to the field of synthesis gas production and to the field of synthesis gas combustion for the generation of power (e.g., generation of electricity) with little or no environmental pollution. In particular, the invention pertains to a closed loop system for the generation and use of synthesis gas for electric power production with zero emissions.
2. Background Information
It is well known in the art that a combustible gas mixture can be produced by the pyrolytic decomposition of a carbonaceous material such as wood, organic refuse, coal and coke. Typically the carbonaceous material is pyrolytically decomposed by contacting hot carbonaceous material with steam under pyrolizing conditions in a vessel. The products of pyrolytic decomposition are mainly hydrogen and carbon monoxide.
It is known to produce a combustible gaseous product which comprises hydrogen and carbon monoxide by the water gas system wherein water or steam is reacted with incandescent carbonaceous material. It is known to use a two-step operation wherein a bed of carbonaceous material, such as coke, is first oxidized by passing air therethrough until the material becomes incandescent and, in the second step, passing steam through the incandescent material to yield the product gasses, including hydrogen and carbon monoxide according to the following chemical equation: EQU H.sub.2 O+C.fwdarw.H.sub.2 +CO
The bed of coke is cooled during the second step, and the first step of air oxidation must be repeated in order to reheat the bed.
It is also known to heat the bed of carbonaceous material electrothermally by using carbon or graphite electrodes. Electrothermic gasification is accomplished by placing the electrodes in contact with the material and applying a sufficient electrical potential to the electrodes, thereby causing resistive heating of the material to sufficiently elevated temperatures which result in the gasification reactions. Water required for the gasification reactions is provided in the form of injected steam or as water vapor from a reservoir located in the bottom of the reactor vessel. In addition to utilizing electrodes for resistive heating, it is also known to carry out the water gas reaction by utilizing an electric arc for heating the material to the required elevated temperatures.
Various technical and economic deficiencies have been noted with respect to the aforementioned prior art technology. U.S. Pat. No. 5,069,765, the specification of which is incorporated herein by reference, is said to provide a more energy efficient and environmentally acceptable method for manufacturing combustible gases from a wide variety of carbonaceous materials. The process described in the aforementioned patent uses a primary reactor, a secondary reactor and optionally a tertiary reactor which are connected in series. A charge of carbonaceous material is fed into the primary reactor which contains electrodes therein for creating an electric arc zone. A constant level of charge is maintained in the reactor and a supply of water for vaporization by the arc is maintained at a level just below the arc zone. When a continuous electric arc is maintained at the electrodes, the intense heat of the arc creates an "arc pocket" in the feed charge at the arc zone, thereby exposing the downwardly feed charge at the periphery of the pocket and the gases and vapors within the pocket to the thermal and photochemical effects of the arc. The primary reactor produces a raw product gas which contains mainly hydrogen and carbon monoxide. It is said that the raw product gas produced in the primary reactor is generally unsuitable for direct use because of its high (approximately 10%) carbon dioxide content.
In order to deal with the undesirable high level of carbon dioxide, the raw product gas is sent to a secondary reactor for reaction with a bed of coke contained therein. The top of the secondary reactor is provided with a single carbon electrode which is positioned within the bed so that the terminal end of the electrode is spaced from the upper level of the coke bed a desired distance in order to permit the creation of an arc between the electrode and the coke bed. In operation, arcing and resistance heating occurs throughout the height of the coke bed which causes the bed to be heated to incandescence. Raw product gas in the secondary reactor is first subjected to the electrothermal and photochemical effects of the arc and thereafter the gas passes downwardly through the incandescent coke bed for further reaction. This results in a reduction in the carbon dioxide content of the product gas. As noted above, it is desired to reduce the carbon dioxide content of the product gas because the gas coming from the primary reactor is unsuitable for direct use because of its high carbon dioxide content. It is said that the refined product gas having a low carbon dioxide content is suitable for combustion in a power generating plant. Thus, it is clear from the disclosure of this patent that the carbon dioxide content of the gas which is burned in the power plant must be minimized.
All gasification processes such as mass burn, incineration, fluidized bed as well as the process described in U.S. Pat. No. 5,069,765 must deal with the problem of removing the inert ash products from the gasifier. In all of the gasification processes in use today, large amounts of ash products, clinkers, etc., fall down onto a metal conveyor or screw system. This allows the removal of the ash from the gasifier to an ash holding compartment where the products are allowed to cool. Costly equipment is generally required to remove pollutants from the ash. After the removal of pollutants from the ash, the ash may then be disposed of in a landfill. Often, the ash products are not completely reacted and there can be as much as about 37% by weight of these ash products left over from the un-reacted feed material.
It is known to remove the ash products from the gasifier and then convey the ash to another vessel that is equipped with electrodes which are adapted to heat the ash to form a molten product which is poured into molds where it is allowed to cool into a glass-like substance. This process is called "vitrification". However, this process is quite cumbersome and there is a possibility that pollutants can be released into the atmosphere unless costly additional equipment is used during the vitrification process. It would therefore be highly desirable to adapt the primary reactor, such as the primary reactor of U.S. Pat. No. 5,069,765 so that the desired vitrification process can be conducted within the gasification vessel so that all potential pollutants which are released from the ash during the vitrification process can remain in the system for further breakdown when subjected to the high temperature pyrolysis conditions. It would also be highly desirable to provide a primary gasifier which avoids the problems associated with clinkers which fall down onto the aforementioned metal conveyor or screw system.
Although the refined product gas produced in accordance with U.S. Pat. No. 5,069,765 is highly refined, the power plants in which this type of gas is combusted typically use air to support the combustion. It is well known that when ambient air or atmospheric air is used for combustion, various types of pollutants such as oxides of nitrogen (NO.sub.x), carbon monoxide and huge amounts of CO.sub.2 are released into the atmosphere. These unwanted pollutants can be removed by the use of various types of catalytic converters or by the use of other costly gas cleaning equipment to meet EPA standards. Thus it would be highly desirable to produce synthesis gas and burn it for the production of power without releasing these or other pollutants into the environment.
In most combustion processes where ambient air or atmospheric air is used, the combustion mixture includes a mixture of gases which are naturally found in the atmosphere. These gases include nitrogen, oxygen, argon and other small amounts of inert gases. During combustion, air enters into the combustion chamber along with a suitable amount of fuel. This fuel can be either liquid or gaseous. The fuel/air mixture is typically compressed and ignited for combustion. The products of combustion are released into the atmosphere. Before being released into the atmosphere, however, considerable cleaning such as by catalytic conversion is necessary to meet emission standards.
The most abundant gases which are used in the above described prior art combustion process are nitrogen and oxygen. The oxygen is necessary as an oxidizer for reaction with the fuel to produce large quantities of heat. This reaction would be quite rapid and could cause severe damage to any engine or power plant if it were not for the large quantities of nitrogen which are present in the atmospheric air. In particular, the nitrogen is considered to be desirable in the combustion process due to the fact that it is rapidly heated and therefore expands and aids in the energy output of the engine or power plant. If the nitrogen were not present, an uncontrollable explosion would occur due to the rapid reaction of oxygen with the fuel. Thus, nitrogen is typically included in the oxidizing gas mixture even though it presents problems with respect to pollution. It would therefore be highly desirable to provide a gasification procedure in which the synthesis gas which is produced can be safely and efficiently combusted for the production of power or heat without causing the aforementioned damage and pollution.
It is also known that currently available gasification processes, such as coal gasification and natural gas generating plants, and combustion procedures used in typical coal-fired generating plants, are faced with the difficult task of cleaning the stack gases to meet stringent EPA regulations. Typically, in these procedures, costly scrubbing and pollution reducing equipment is necessary to treat the stack gases before they are released to the atmosphere. A gasification and combustion process which does not produce any stack gases would therefore be highly desirable.
Also, conventional gasification and combustion processes typically produce contaminated cooling waters and scrubbing waters as well as sludges which cannot be discharged into the environment without harming the environment. Therefore, a gasification and combustion process which does not require discharging cooling water, scrubbing water and sludges into the environment would be highly desirable. In particular, it would be highly desirable to produce a gasifier in which the ash can be efficiently and safely vitrified within the gasifier and discharged therefrom without releasing undesirable pollutants into the atmosphere. In addition it would be highly desirable to produce a gasification and combustion process in which stack gases are eliminated and in which materials such as sludge, cooling water and scrubbing water are recycled through the system so that the system produces zero emissions and zero water pollution. Until the present invention, no one has developed such a system which effectively deals with all the aforementioned problems and environmental concerns.
Various types of gasifiers and/or combustion systems are known in the prior art. For example U.S. Pat. No. 233,860 discloses a gasifier which includes a bottom portion for the collection of slag and molten metal therein. An upper tap is provided for withdrawing molten slag from the device and a lower tap is provided for withdrawing molten metal from the device. The device also includes a steam injection pipe for the introduction of steam into the reactor.
U.S. Pat. No. 2,593,257 discloses a furnace which collects molten slag and metal at the bottom portion thereof. The device includes a discharge outlet for removing slag and a slightly lower discharge outlet for discharging molten metal.
U.S. Pat. No. 2,163,148 discloses a water-gas generator which includes a bottom portion for the collection of molten slag. The device also includes a discharge conduit for removing the molten slag from the reactor.
U.S. Pat. Nos. 5,430,236; 4,188,892; 4,666,490; 5,603,684; 5,950,548 and 4,180,387 disclose the vitrification of ash to produce an ash product which can be safely disposed of.
U.S. Pat. No. 5,724,805 discloses a power plant which is operated by combusting gaseous fuel such as synthesis gas with substantially pure oxygen as an oxidizer in the presence of carbon dioxide as a diluent. Carbon dioxide produced during combustion is recirculated for use as a diluent gas. It is said that the process emits virtually no pollutants due to the use of carbon dioxide as a diluent and substantially pure oxygen as the oxidizing gas.
U.S. Pat. No. 3,866,411 discloses combining a process for producing synthesis gas with a combustion procedure wherein the gas is burned for the production of power. The combustion portion of the process optionally uses substantially pure oxygen as an oxidizing gas in the presence of flue gas produced in the process. The flue gas includes carbon dioxide. Of similar interest is U.S. Pat. No. 3,868,817.
U.S. Pat. No. 4,881,366 teaches that burning carbon monoxide with oxygen reduces emissions of nitrogen oxides.
Additional patents which are relevant to the general technical field of this invention include U.S. Pat. Nos. 5,135,361; 5,177,952 and 5,595,059.